Cyclic neutropenia, or cyclic hematopoiesis, is a disease of humans and collie dogs caused by a marrow deficit on regulation of blood cell formation. On the basis of transplantation experiments, it is presumed to be due to a stem cell abnormality. In the coming year we will extend our studies on the patterns of fluctuation of CFU-C and CFU-E in these animals with and without endotoxin treatment, study certain cell cell interactions which may lead to the cycling of the marrow and blood cells and perform studies on agents other than endotoxin which may alter cyclic hematopoiesis.